Current art for the production of cross-linked polyolefin foam structures involves the use of conventional high-pressure reactor-produced, low density polyethylene (LDPE). LDPE is comprised of a wide-ranging distribution of branch lengths, best characterized as "long- but variable- chain branching", and a molecular weight distribution (Mw/Mn) which is generally greater than about 3.5. LDPE resin densities, which directly relate to the resulting bulk property stiffness, typically range between 0.915 to about 0.930, thus limiting the degree of mechanical flexibility in foam structures thereof since the lower limit of secant moduli for LDPE is about 20 ksi. While processability of LDPE is quite good, the physical properties, in particular the tensile strength, low-temperature flexibility and toughness, are less than would be obtained from a linear low density polyethylene (LLDPE), due in part to the substantially non-linear nature of LDPE and the profusion of "long-chain branches."
Conventional linear low density polyethylene (LLDPE) exhibits physical properties which are superior to that of LDPE at about the same range of resin densities, but show somewhat higher secant moduli and are difficult to process, resulting in foams with poor cell structure and higher than desired foam structure densities. LLDPE resins, produced by conventional Ziegler transition metal catalysts in the copolymerization of ethylene with one or more alpha-unsaturated monomers, exhibit considerably narrower molecular weight distributions than LDPE, higher molecular weights, and typically only about 15-20 "short-chain branches" per 1000 carbon atoms. Melt-processing in general, and foam processing in particular, are greatly enhanced by the ability of the resin to "shear-thin" or demonstrate a strong, inverse dependency of melt viscosity toward shear rate. "Shear thinning" increases with the degree of branching, which is exemplified in the relative shear-insensitivity of LLDPE and particularly HDPE and resulting foam processing difficulty. Commercially-available LLDPE resins with densities below about 0.910 g/cc are unavailable, thus further limiting the flexibility of foam structures thereof.
Very low density polyethylene (VLDPE) is a special subset of LLDPE wherein an even greater number of "short-chain branches" (ca. 30-50 per 1000 carbon atoms) are effected by appropriate level of comonomer to result in much lower resin densities than LLDPE, e.g. 0.88 g/cc to 0.91 g/cc. These materials thus exhibit greater flexibility than LLDPE. However, generally with conventional linear polyolefins, the greater the number of "short-chain branches," the lower the resulting resin density, but also the shorter the length of the molecular backbone. The presence of shorter molecular backbones, with greater comonomer content therein, prematurely leads to a phenomena known as "melt fracture," which is manifested as the onset of perturbations at the surface of an extrudate with increasing shear rate, resulting in loss of control of the quality of such profiled, extrudable materials.
Certain other undesirable structural features accompany efforts to increase "short-chain branching" while employing conventional linear polyethylene technology, such as an increase in the non-uniformity of the distribution of branches on the molecular backbone. Additionally, conventional linear polyethylene technology leads to a distribution of molecular weights, with a greater propensity of incorporation of the alpha-unsaturated comonomer into the lower molecular weight fractions, thereby leading to melt fracture. Also, as a result of this non-uniformity of molecular weights and distribution of comonomeric species within and among the distribution thereof, linear polyolefins exhibit less than optimal performance in various parametric standards such as toughness, particularly at low temperatures, and stability of extrusion, particularly at high rates.
Many of the above noted deficiencies in the foamable polyolefin art could be satisfied through the use of a linear olefinic resin which is essentially free of "long-chain branches", and which has a molecular weight that is sufficiently high to preclude melt-fracture, a narrow molecular weight distribution, a considerable melt-viscosity/shear rate sensitivity and a full range of resin densities. Such a linear polyolefin would exhibit the preferred balance of physical properties, would exhibit good toughness and processability, and would be available in a range of resin flexibilities. It is thus an object of this invention to provide a linear olefinic resin which possesses these characteristics.
Various catalysts are known to the art of polyolefin foams. "Metallocenes" are one class of highly active olefin catalysts, well known in the art of preparation of polyethylene and copolymers of ethylene and alpha-unsaturated olefin monomers. U.S. Pat. No. 4,937,299 (Ewen et. al.) teaches that the structure of the metallocene catalyst includes an alumoxane which is formed when water reacts with trialkyl aluminum with the release of methane, which complexes therein with the metallocene compound to form the active catalyst. These catalysts, particularly those based on group IV B transition metals such as zirconium, titanium and hafnium, show extremely high activity in ethylene polymerization.
Metallocene catalysts have great versatility in that, by manipulation of process conditions such as catalyst composition and reactor conditions, they can be made to provide polyolefins with controlled molecular weights from as low as about 200 to about 1 million or higher. Exemplary of the latter case is ultra-high molecular weight linear polyethylene. At the same time, the molecular weight distribution of the polymers thereof can be controlled from extremely narrow to extremely broad, i.e. from less than 2 to greater than 8.
Metallocene catalysts are particularly advantageous in the preparation of copolymers of ethylene and one or more alpha-unsaturated olefin comonomers to provide highly random distributions of comonomer within each and every molecular backbone, while separately controlling the average molecular weight as well as the distribution of molecular weights about the average. It is thus an object of the present invention to use the versatility of metallocene catalysts to produce linear olefinic resins having the aforementioned properties.
These and other objects are realized by the present invention, as disclosed herein.